Milka Avramov-Ivić

The electrocatalytic properties of the oxides of noble metals in the electro-oxidation of some organic molecules☆

The electro-oxidation of organic compounds on gold and silver electrodes can occur either in a potential range more negative than that where the metal oxides are formed or within the more positive range where the surface is covered by oxides. Oxidation of methanol on gold electrodes proceeds in the potential range where the electrode is not covered with oxide and only at the slowest scan speed used (0.1 mVs−1). At the same scan rate the Au(110) orientation as well as the Au(111) orientation favor the catalytic activation of the anodic electro-oxidation of methanol in the region of oxide formation. Oxidation of malic acid on a gold electrode proceeds only in the region where the electrode is covered by gold oxide. On a glassy carbon electrode modified by silver, reactivation of the formaldehyde anodic oxidation is observed in the region where the electrode is covered by silver oxide. On the bulk silver electrode and on the glassy carbon electrode modified by silver, alcohols such as methanol, ethylene glycol and glycerol are oxidized only in the region where the electrodes are covered by silver oxide. On such electrodes the observed anodic currents increase in the sequence: methanol < methylene glycol < glycerol, and in the same sequence the corresponding peaks occur at more negative pot

Reduction of carbon dioxide on ruthenium oxide and modified ruthenium oxide electrodes in 0.5 M NaHCO3

Abstract Electrochemical reduction of carbon dioxide is still of great interest in the fields of theoretical and applioed electrochemistry Ruthenium is one of the metals with an intermediate hydrogen evolution overpotential which adsorbs a considerable amount of hydrogen at underpotentials. Therefore, one can expect that the reduction of CO 2 will take place at this electrode at relatively small overpotentials, at which the hydrogen evolution will not be excessive. This will diminish the loss due to concurrent hydrogen evolution. The stability of the electrode surface under vigorous hydrogen evolution enables investigation of the kinetics of CO 2 reduction under long-term potentiostatic conditions. Reduction of CO 2 at ruthenium and ruthenium modified by Cd and Cu adatoms was investigated in 0.5 M NaHCO 3 solutions. Cyclic voltammetry, potentiostatic measurements and long-term electrolysis were performed using standard electrochemical equipment. The analysis of the bulk of the electrolyte was performned with gas chromatography. On the Ru surface as well as on Ru modified by Cu and Cd adatoms and on RuOx + IrOx modified by Cu adatoms and Cd adatoms, CO 2 reduction proceeds to give methanol and acetone during 8 h of holding the potential at −0.8 V. The process of methanol formation on Ru modified by Cu and Cd adatoms is catalyzed by the presence of adatoms and the process of acetone formation is independent of the presence of adatoms at the ruthenium surface. For all the examined surfaces and for further investigations the most promising fact is the relatively low observed overvoltage of the holding potential for CO 2 reduction, at which methanol and acetone were found in the bulk of the electrolyte. The faradiac efficiencies obtained for methanol production in the case of modified ruthenium surfaces indicate some possibility of practical application.

Structural effects in electrocatalysis: oxidation of formaldehyde on gold and platinum single crystal electrodes in alkaline solution

The oxidation of formaldehyde in sodium hydroxide solution has been studied on platinum and gold single crystal electrodes with the (111), (110) and (100) orientations. There is apparently no structural sensitivity of this reaction, since minor differences have been found between the three low index faces. This is valid for both platinum and gold electrodes. The hydrogen adsorption on platinum and the AuOH formation on gold electrodes exhibit structural sensitivity in the same solution. Similar activity of platinum and gold electrodes is noteworthy. A weak adsorption of gem-diol, formed in the interaction of formaldehyde with H2O or OH− appears as the origin of the structural insensitivity of this reaction.

The electrocatalytic properties of the oxides of noble metals in the electrooxidation of methanol and formic acid

The electrooxidation of 0.25 M methanol as well as of 0.25 M formic acid was examined on oxides formed on the Au( 111) electrode surface in 0.5 M H 2 SO 4 and 0.5 M NaHCO 3 at a sweep rate of 0.5 mV/s. In the case of methanol, the Au( 111) surface in 0.5 M H 2 SO 4 must be covered by oxides in order to catalyse its oxidation significantly. The current of formic acid oxidation also appears in the potential region before the oxide formation, but the catalytic activity is apparently deeper in the potential region of oxide formation. In 0.5 M NaHCO 3 the inhibition of methanol oxidation and the apparent catalytic activity of Au oxides in the case of formic acid oxidation are observed. The methanol electrooxidation on polycrystalline silver electrode was observed only in the potential region where the surface was covered by oxides in 0.1 M NaOH and at a sweep rate of 0.1 mV/s. This reaction was used to measure the concentration of methanol in the methanol-formaldehyde mixture in the same electrolyte.

The electro-oxidation of glycerol on the gold(100)-oriented single-crystal surface and poly crystalline surface in 0.1 M NaOH

In this work, the oxidation of glycerol on a gold single-crystal surface with the (100) orientation has been studied with the aim of investigating the interfacial properties of the electrode surface in the absence and presence of glycerol. The change in the interfacial properties gives useful information about the nature of OH adsorption on a Au(100) electrode and about the process of adsorption and faradaic oxidation of glycerol in 0.1 M NaOH. The electrosorption of glycerol was also investigated at a polycrystalline gold electrode, by programmed potential voltammetry, in order to confirm the results obtained by double-layer capacity measurements

An investigation of the oxidation of formaldehyde on noble metal electrodes in alkaline solutions by electrochemically modulated infrared spectroscopy (emirs)

Abstract In-situ spectroscopy was used to detect adsorbed species formed during the oxidation of formaldehyde at polycrystalline Au, Pt, Rh and Ir electrodes in alkaline solution. At the gold electrode, the spectra showed the oxidation of the gem-diol form of formaldehyde and the formation of an adsorbed formate species. At platinum, rhodium and iridium, adsorbed CO was observed in the hydrogen adsorption region where the oxidation of formaldehyde is strongly inhibited on these metals. Alternative routes for producing the poison are discussed.

SYNTHESIS, CHARACTERIZATION, STRUCTURE AND POSSIBLE CATALYTIC PROPERTIES OF CIS-OXALATO(1,4,8,11-TETRAAZACYCLOTETRADECANE)COBALT(III) NITRATE

Abstract A new complex of cobalt(III) with 1,4,8,11-tetraazacyclotetradecane (cyclam) and oxalato ion as a bidentate ligand was prepared and characterized by elemental analysis, IR, electronic and 1H NMR spectroscopy and cyclic voltammetry. X-Ray analysis has shown that this compound crystallizes in the orthorhombic system, space group Pccn, with a = 8.583(1), b = 12.854(2), c = 14.944(1)A, V = 1649.3(4) A3, Z = 4, R = 0.512, Rw = 0.545, and has a crystallographic two-fold rotation axis. The complex was identified as cis-oxalato(1,4,8,11-tetraazacyclotetradecane)cobalt(III) nitrate, [Co(ox)cyclam]NO3 (oxH2 = oxalic acid), and it can be described in terms of a cis octahedral geometry with a folded cyclam configuration around the cobalt atom with the oxalato ion occupying the remaining two sites. Cyclic voltammetric data suggest a large stability for this compound, as well as its possible catalytic effect on electrochemical CO2 reduction.

Structural effects in electrocatalysis: Ethylene glycol oxidation on platinum single-crystal surfaces

Abstract The oxidation of ethylene glycol (EG) has been studied on platinum single-crystal surfaces in 0.1 M NaOH. A strong structural dependence of the reaction kinetics was found for all 12 single-crystal orientations investigated. The onset of the reaction occurs in the sequence Pt (110) > Pt (100) ≈ Pt (111). However, the peaks of the voltammetry curves for the low-index planes decrease in the order (111) > (110) > (100). Either the (111) or the (100) oriented steps cause a decrease in the activity of the (111) plane. Surfaces vicinal to the (100) plane exhibit a higher activity than Pt (100). The (111) oriented steps in the Pt (110) plane cause an increase in activity, while the (100) oriented steps decrease it. A parallelism between PtOH layer formation and the onset of the oxidation of EG was found which indicates that the reaction involves interaction of the adsorbed and dehydrogenated EG with Pt(OH)ads species.

Preparation, characterization and redox properties of o,o′‐geometrical isomers of μ‐α and μ‐β‐aminoisobuty‐rato co(ii) complexes with n,n′,n′,n‴‐tetrakis(2‐pyridylmethyl)‐1,4,8,11‐tetraazacyclotetradecane

Abstract Two new dinuclear cobalt(II) complexes which, besides macrocyclic ligand N′,N′,N″,N‴-tetrakis (2-pyridylmethyl)-l,4,8,11-tetraazacyclotetradecane (tpmc) contain α- or β-aminoisobutyrato ions (α-aibu − and β-aibu −, respectively) were synthesized. The complexes having general formula [Co2(aibu)tpmc](CIO4)3 were characterized by elemental analysis, electronic and 1R spectroscopy, magnetic measurements and cyclic voltammetry. It was assumed that in both complexes an exo coordination of the macrocyclic ligand is achieved. Cobalt(II) ions are bridged with O,O′-aminoisobutyrato ligand. Both complexes are stable against air oxidation, whereas the oxidation of the complex containing the α-aibu− ion by hydrogen-peroxide gave the coresponding mixed-valence Co11/Co111 complex. The complexes are also electrochemically stable in the potential range from-0,6V to 0,4V vs SCE.